1. Field of the Invention
The present invention relates to olefin polymerization. In particular, the present invention concerns a process for polymerization of propylene in a polymerization reactor arrangement to produce at least two olefin polymers having essentially the same level of Melt Flow Rate but different isotacticity.
2. Description of Related Art
Polypropylenes are conventionally produced in the presence of a high-yield catalyst system comprising a catalyst component, a cocatalyst component and a compound acting as an external electron donor. The catalyst component is typically a supported Ziegler-Natta type catalyst containing magnesium, titanium and a halogen as essential components. Porous, inorganic or organic particulate carrier materials, such as silica or MgCl2 supports, are used as support materials. The external electron donors present a means for controlling the isotacticity of the polymer.
For different applications, polymers with very different properties are required. The main characteristics of these polymers are their isotacticity and Melt Flow Rate, in the following also abbreviated “MFR”. These features can be controlled by varying the process conditions and by using different catalyst systems. One important tool for adapting the properties of the polymers to varying product requirements is adjustment of hydrogen feed during polymerization. By varying of the feed of hydrogen, the molecular weight or MFR of the polymer can be controlled. The stiffness of the polymer is also an important product property, which should be adjusted depending on the end use of the polymer. Stiffness is greatly dependent on the isotacticity of the polymer and, thus, isotacticity has to be set at the desired, predetermined level. This is typically done by proper selection of the external donors used in the polymerization process.
It is commonly known that different external donors lead to polymers with different isotacticities and to polymers having different mechanical properties. One of the important mechanical properties is the flexural modulus. It is also known that the isotacticity can be affected to some extent by changing the concentration of the donor. This change in donor concentration influences the concentration of xylene-solubles of the polymer, which is usually not desired.
There are problems related to the known processes, when transitions from one polymer grade to another grade have to be made. It is often necessary to produce different polymer grades with the same process equipment. With prior art processes it is extremely difficult, and sometimes even impossible, to change over from one polymer grade to another by changing one polymer characteristic without affecting the other polymer characteristics. According to the prior art, the change in one feature or one component of the process tends to cause changes in other features, too. This means in practice that if one component or feature in a process for changing one property of the product is changed, then one or more other components of the process have to be changed, too, if the other properties of the polymer are to be kept unchanged. This is due to the fact that components of the process are so closely linked together that changes in one component mean changing the others, too. This is a great problem with present-day processes, where the transition of polymer grades having different stiffness, but a predetermined MFR has to be made.
As is stated above, different isotacticity levels require the use of different external donors. However, hydrogen response of the catalyst system is dependent on the type of external donors used. “Hydrogen response” or “hydrogen sensitivity” stands for the sensitivity of the molecular weight of the polymer to the hydrogen concentration. This means that if the external donor is changed, the hydrogen feed to the process needs to be recalculated and changed in order to meet the requirements for a specific level of melt flow rate. As far as the process is concerned, this causes extra work, time and costs.
Thus, in summary, according to known processes, when isotacticity and MFR of the polymer are modified, the donor needs to be changed and the hydrogen flow to the process needs to be altered. Even if only isotacticity is to be changed by changing the donor, also the hydrogen feed has to be changed due to the reasons explained above. Further, there is always a transitional period of time before the grade of the polymer product is changed from a first polymer grade to another, since conventional processes stabilize slowly after a change of donor and hydrogen feed. The material produced during the switching-over period between two grades often has to be discarded because it does not fulfil the quality requirements neither of the first nor of the second polymer. This is both an economical and an environmental disadvantage.
Until now, no method of transition from one polymer grade to another is known, wherein the hydrogen feed could be kept on the same level when an external donor is changed, in order to change the isotacticity, but to keep the MFR at the predetermined level. In practice, this means that there are no processes that would allow for facile transition from one polymer grade to another having different isotacticity but essentially the same MFR.